Many lasers used in today's markets employ solid state gain mediums. These mediums can be made of a number of materials, doped with a variety of elements. One of the less desirable properties of these materials is that they tend to form thermal lenses when pumped. The thermal lens is brought about by the fact that in general all of the light used to pump the medium is not absorbed directly into the pump bands therefore a considerable amount of energy is released to the lattice by non-radiative decay leading to heat. The lens can either be positive or negative depending upon whether the medium has a positive or negative dn/dT, where n is the idex or refraction and T is the temperature. Most of the more common materials used today have a positive dn/dT thus when thermal equilibrium is established the medium will act as a positive lens with predictable characteristics and can be used as a stable element of a laser cavity.
The behavior of the laser during the build up to steady state of the thermal lens can cause problems affecting the intra-cavity optics. With the initial pump pulse from the flashlamps a larger portion of the pump light is absorbed at the surface of the rod than at the center. This will cause the rod to behave initially as a negative lens. Depending on the strength of this lens and the power of the resonator optics the resonator may become initially unstable. Successive pump pulses will continue to heat the rod until a steady state condition arises where the heat flow from the rod will be from the center, thereby reversing the previous condition and forming the stable positive lens. This process can take several seconds thus the initial laser pulses will be continually evolving until the steady state is reached. Holtz, R.F., Thermal Transient Effects in Repetitively Pulsed Flashlamp-Pumped YAG:Nd and YAG:Nd, Lu Laser Material, Applied Optics, Vol. 12, No. 8, (August 1973). If the resonator is unstable during the initial pumping, the rays propagating back and forth between the resonators mirrors will walk out of the cavity, as shown in FIG. 1. The rays will be limited ultimately by some aperture in the laser (possibly the gain medium itself). Diffraction within the resonator will exhibit characteristic Fresnel diffraction effects, the outcome of which can lead to on axis intensity peaks that can be four times the intensity in the limiting aperture. Siegman, A.E., Unstable Optical Resonators, Applied Optics, Vol. --, No. 2, (February 1974). The amplification of these peaks with successive passes through the gain medium can often lead to coating damage to any of the intra-cavity optics. These damage sites need not always be located on the mechanical axis of the resonator. Only in a perfectly aligned resonator with no aberrations will they appear at the mechanical axis. Under these conditions the optic axis is coincident with the mechanical axis. With phase tilt, astigmatic, and higher order aberrations present these damage sites need not be confined to the mechanical axis.
One approach to solving these problems has been to use an intracavity shutter, which blocks the intracavity optical path while the gain medium is pumped to its hot equilibrium condition. Once the thermal lens is formed, the intracavity shutter is opened and the gain medium lases with no undesirable transitional optical scheme. One problem with this approach is that it is undesirable to accessorize the laser with moving parts which create the potential for mechanical failure. Another problem with the intracavity shutter is that laser intracavity space is frequently limited. Thus, it is desirable, if not necessary, to solve the problem without adding additional physical elements to the intracavity space.
Another approach to the problem is to increase the damage threshold of the coatings at the faces of the gain medium. However, this approach only slightly extends the life of the coating without solving the underlying problem. The coating still becomes damaged, causing the coating to fail prematurely.
Therefore, it is an object of this invention to allow a solid state gain medium to come to thermal equilibrium prior to laser emission, thereby avoiding the negative effects of Fresnel diffraction brought about by operation of the laser in an unstable resonator condition.
Other related objects of the invention are to accomplish the objective stated above without using moving parts and without encroaching on or affecting intracavity space.
Another object of the invention is to bring the laser gain medium to thermal equilibrium in accordance with the above objectives, as rapidly as possible.